1. Technical Field
The present invention relates to a method of determining the reliability of long-term predicted orbit data, a method of providing long-term predicted orbit data, and an information providing apparatus.
2. Related Art
A global positioning system (GPS) is widely known as a positioning system employing positioning signals and is used in positioning devices built in mobile phones and car navigation apparatuses. In the GPS, a positioning calculation of calculating a three-dimensional coordinate value indicating the position of a target apparatus and a clock error is carried out on the basis of information such as the positions of plural GPS satellites or the quasi-distance from the GPS satellites to the target apparatus.
In measuring the position using the GPS, satellite information such as the position, velocity, and direction of movement of the GPS satellite is first calculated on the basis of navigation data, such as almanac or ephemeris, which overlaps with a GPS satellite signal emitted from the GPS satellite. The almanac serves as an influential key to capturing a satellite but is not generally used in the positioning calculation, because the satellite information is of poor precision. On the other hand, the ephemeris serves as an influential key to capturing a satellite and can be also used in the positioning calculation, because the satellite information has excellent precision. Therefore, for example, when the positioning calculation is started without the ephemeris, the ephemeris should be acquired from the GPS satellite signal, thereby enhancing the TTFF (Time To First Fix).
Therefore, as a server and client system, a technique of allowing a server to predict long-term predicted ephemeris (long-term predicted orbit data), which is the ephemeris corresponding to a term of one week, and to provide the long-term predicted ephemeris to a positioning device as a component has been developed and disclosed, for example, in US-A-2002-188403 and US-A-2005-212700.
A method of defining the long-term predicted ephemeris in the same data format as a typical ephemeris can be considered as a method of defining the long-term predicted ephemeris. That is, a satellite orbit is approximated using Keppler's elliptical orbit model which is one of satellite orbit approximate models and the long-term predicted ephemeris is defined using values of parameters (hereinafter, referred to as “satellite orbit parameters”) of the model expression. A predicted satellite calendar (predicted position data) including predicted positions acquired by predicting future positions of a positioning satellite at a predetermined time interval in time series is provided from a predetermined commercial system. The approximating calculation, using Keppler's elliptical orbit model, can be carried out using the predicted satellite calendar.
However, the predicted positions of the positioning satellite included in the predicted satellite calendar tend to be mismatched with the actual positions of the positioning satellite as it moves into the future. Accordingly, when the long-term predicted ephemeris is created by carrying out the approximating calculation using Keppler's elliptical orbit model, the satellite orbit calculated by the approximating calculation tends to be mismatched with the actual satellite orbit as it moves into the future from the creation time. In the past, since there was no technique of determining the reliability (whether it is suitable for positioning) of the created long-term predicted ephemeris after the long-term predicted ephemeris is once created, the positioning device having acquired the long-term predicted ephemeris from the server might carry out the positioning calculation using the long-term predicted ephemeris with low reliability.